U.S. Pat. No. 4,446,221 which issued to Herting et al. is directed to an improved lithographic printing plate which comprises treating an anodized metal plate with an aqueous solution of an alkali metal salt of a condensed arylsulfonic acid having an acidic pH prior to coating the metal plate with a radiation-sensitive composition. Herting et al. discuss in columns 1 and 2 of the patent certain problems encountered in the prior art methods for coating a radiation-sensitive materials onto a lithographic plate. It is stated there that radiation sensitive elements which may be converted to lithographic printing plates comprise a radiation sensitive layer in which the printing image is photomechanically produced; and a suitable support which from the production of the material until its processing into a printing plate, carries the radiation-sensitive layer and is stored therewith until the material is used. After the production of the printing image, the support carries the printing image and simultaneously forms the image background in the image-free areas. A suitable support for a printing plate is one where the printing image areas developed from the radiation-sensitive layers must adhere very firmly to the support. Furthermore, the support must have a hydrophilic surface, and the repelling effect thereof with respect to oleophilic printing inks must not decrease under the multiple printing commercial requirements. The support should have a surface structure which is porous so that the surface can retain sufficient water to have an adequate oil repelling effect with respect to the printing inks.
Aluminum oxide layers prepared by anodic oxidation of aluminum sheets or foils are extraordinarily abrasion resistant, and such anodized sheets have been found to be very useful in the production of long running printing plates. However, such plates suffer from disadvantages caused by too great a penetration of the radiation-sensitive composition into the pores of the anoidic oxide support.
In the past such disadvantages have been obviated, for example, by treating the anodized support with an aqueous solution of sodium silicate, ammonium or alkali bichromate, iron ammonium oxalate or a dyestuff which can react chemically with the aluminum oxide surface prior to coating the latter with the radiation-sensitive composition.
Such processes also suffer from certain disadvantages. Thus, treatment with alkali metal silicate entails the requirement of thorough rinsing with water when the support is to be provided with a storable radiationsensitive layer adapted to be stored over a long period without deterioration of the radiation-sensitive layer. Even after thorough rinsing with water or neutralization with dilute acids, the alkali metal silicate layer or perhaps the alkali residue remaining from the silicate solution may undergo undesirable degradation. When aqueous chromate solutions are used, a barely hydrophilic intermdiate layer is obtained. Furthermore, the use of chromium-containing compositions is now considered environmentally undesirable. Similar considerations, except for the environmental, apply to treatment with an iron ammonium oxalate solution, the iron content of which may cause a dark discoloration.
When using aqueous solutions of dyestuffs that chemically react with the aluminum oxide surface, the hydrophilic properties of the aluminum oxide layer are reduced. Consequently, a printing plate material produced with a support treated in this manner tends to scum more during printing. In addition, the aluminum oxide surfaces chemically altered by means of dyestuffs cannot be easily corrected.
The patent to Herting et al. constituted an improvement over the prior art processes insofar as it treated an anodized support material with an alkali metal salt of a condensed arylsulfonic acid, such as Tamol SN, having an acidic pH. It has now been found that the aqueous solution having the acidic pH has to be constantly monitored and maintained by adding by the addition of an acid such as sulfuric or phosphoric acids. It was also found that in an acidic aqueous solution of the alkali metal salts of condensed arylsulfonic acids the pH of the solution increased significantly when the alkali metal salt concentrations decreased while operating at higher temperatures and over an extended period of time, e.g. from 11 to 75 hours or longer.
For certain purposes, it would be helpful to have available a process of applying the alkali metal salt of the arylsulfonic acids to the lithographic metal plate which did not require the monitoring and maintenance when one utilizes an acidic aqueous solution of such alkali metal salts. It would also be helpful in this connection to have available a method where the pH did not increase rapidly during the processing conditions.